Refineries and petrochemical plants in the United States use 40,000 distillation columns to separate organic liquid mixtures. These columns account for approximately 3% of total U.S. energy consumption.
In principle, these separations could be performed at a much lower cost and with far less energy consumption by permeation of the liquids through membranes.
Interest in using pervaporation for separating organic mixtures has waxed and waned over many years. The first systematic studies of pervaporation for separating mixtures of aromatics, or aromatics from aliphatics, were performed by Binning, Lee, Stuckey and others at American Oil in the 1950s. This work is exemplified in U.S. Pat. No. 2,930,754 and other similar patents.
In the 1970's, work on similar separations was carried out by Perry and others at Monsanto. Patents assigned to Monsanto disclose a variety of pervaporation applications. For example, U.S. Pat. No. 3,966,834 concerns separation of dienes from mono-unsaturated compounds.
In the late 1980's and early 1990's, various oil companies—Texaco, Mobil, and particularly Exxon—undertook significant research programs to develop improved membranes and processes for use in aromatic/aliphatic separations. For a few years, Exxon was the most prolific patentee in any membrane-related area on the strength of this effort. Exemplary patents to Schucker and others in this period include U.S. Pat. Nos. 4,929,358 and 5,290,452.
The separation of olefins from paraffins is an organic/organic separation of particular importance. Olefins, particularly ethylene and propylene, are important chemical feedstocks. About 17.5 million tons of ethylene and 10 million tons of propylene are produced in the United States annually. Before they can be used, the raw olefins must usually be separated from mixtures containing saturated hydrocarbons and other components.
Because olefins and the corresponding paraffins are similar in molecular size and condensability, their separation with polymeric membranes is very difficult. For these separations, much effort over the years has been devoted to developing facilitated-transport membranes. Such membranes use a carrier, usually a silver salt solution, that is held in a polymeric matrix and selectively complexes with the olefin. Although these membranes can exhibit high olefin/paraffin selectivity, they tend to be very thick, making fluxes undesirably low, and suffer from instability problems that can degrade performance in only hours.
Fluorinated polymers, especially fluorinated polyimides, have a reputation for thermal and chemical stability. It has been attempted to use fluorinated polyimides for separation of organic liquid mixtures. U.S. Pat. No. 5,749,943, to Petroleum Energy Center of Japan, describes separation of light olefins from paraffins using membranes made from specific fluorinated polyimides. The patent claims gas-phase separations, but mentions that the method can be carried out by pervaporation.
U.S. Pat. No. 5,112,941, to Mitsubishi Kasei discloses the treatment of aromatic polyimide membranes by exposure to fluorine gas to increase the membrane selectivity. The patent mentions that the membranes would be suitable for use in pervaporation.
U.S. Pat. No. 5,153,304, to Sagami Chemical Research Center, discloses polyimides with fluorine-containing groups in the side chains, and gives an example of the use of the polymers as pervaporation membranes.
Despite their relatively good chemical resistance, fluorinated polyimide membranes have not been commercialized for pervaporation separations. When exposed for long periods to aggressive hydrocarbons, they tend to plasticize and lose their separation capabilities. Also many polyimide structures are extremely rigid, and offer low permeability, so that membranes made from them provide only low transmembrane flux, making them impractical when large volumes of feed are to be processed.
Other fluorinated polymers have also been considered for use in pervaporation. U.S. Pat. No. 4,666,991, also to Sagami Chemical Research Center, discloses graft copolymers having a fluorinated acrylate as the graft polymer, and mentions that the copolymers are useful for pervaporation of organic liquids, although the only data given in the patent refer to ethanol/water separations.
U.S. Pat. No. 5,387,378, to Tulane University, describes asymmetric membranes made from polyvinylidene fluoride polymers and copolymers. The patent shows experimental data, mostly for separation of organics from water, but, in one case, for separation of benzene/cyclohexane or toluene/ethanol.
U.S. Pat. No. 5,396,019, to Exxon, describes separation of toluene from n-octane, as well as other aromatic/aliphatic separations, using membranes made from crosslinked fluorinated polyolefins, such as polyvinylidene fluoride or polytrifluoroethylene.
Ion-exchange, or ionic, membranes, contain charged groups attached to the polymer backbone of the membrane material. These fixed charge groups partially or completely exclude ions of the same charge from the membrane. Among the best known ion-exchange membranes are those sold under the name Nafion®. These membranes comprise a polymer of a perfluorosulfonic acid or a derivative thereof.
Such membranes, more commonly used for electrodialysis, have been suggested for certain pervaporation applications, usually involving water/organic separations. A representative application of that type is disclosed in U.S. Pat. No. 4,876,403, to Exxon.
Because of their charged nature, ion-exchange membranes are essentially impermeable to hydrophobic hydrocarbons, and are not suitable for separating mixtures of such compounds. A few mentions of the use of ion-exchange membranes for separating more polar from less polar organics occur in the literature. U.S. Pat. No. 5,238,573, to Texaco, describes such a process. Nafion® membranes in which the hydrogen atoms of the acid group have been replaced by metal cations are used to separate water, methanol or other light alcohols from an oxygenate, such as an ether or ester. The small, highly polar water or alcohol molecules can permeate the membrane; the more hydrophobic components are retained.
Despite this wealth of research, both in the laboratory and in pilot plants, membranes and processes able to stand up to industrial conditions, and to be technically and economically competitive with distillation, have not been available to date.
Until recently, there was also a long-felt need for gas separation membranes able to withstand exposure to organic vapors, such as C3+ hydrocarbons, that might be present in the gas mixture to be separated.
U.S. Pat. Nos. 6,361,582: 6,361,583; and 6,271,319 co-owned with the present application, and incorporated herein by reference in their entirety, describe gas separation processes that use organic-vapor-resistant membranes.
In U.S. Pat. No. 6,361,583, the membranes are made from glassy polymers or copolymers characterized by having repeating units of a fluorinated, cyclic structure, and having a fractional free volume lower than 0.3 and a glass transition temperature of at least about 100° C.
In U.S. Pat. No. 6,361,582, the polymer need not contain a ring structure, but is heavily fluorinated, having a fluorine:carbon ratio of atoms in the polymer of at least about 1:1.
In U.S. Pat. No. 6,271,319, processes for improving manufacture of polypropylene are described. The processes use a membrane separation step to recover propylene from an overhead vent gas stream. The membranes may be made from various materials, including fluorinated dioxoles and cyclic ethers.
U.S. published patent application number 2002/0065383, and corresponding U.S. Pat. No. 6,469,116, to Ausimont, describe manufactured articles, including separation membranes, made from the types of polymers preferred in U.S. Pat. No. 6,361,583.
U.S. Pat. No. 6,316,684, co-owned with the present application, describes gas or liquid separation processes suitable for separating organic components from mixtures. The processes use filled membranes, that is membranes made from a polymer matrix of relatively high free volume, with a very fine non-porous filler material dispersed in the polymer matrix. The membranes exhibit unexpected combinations of high flux and high selectivity for certain separations. One polymer used as the matrix material is perfluorinated poly 2,2-dimethyl-1,3-dioxole (Teflon® AF).